1. Field of the Invention
The present invention relates to a galvannealed steel sheet having excellent surface appearance and press formability and a method therefor.
2. Description of the Related Art
Recently, galvannealed steel sheets have been widely used to manufacture bodies of automobiles in order to improve the corrosion resistance. Since the galvannealed steel sheets have been used to manufacture also the outer body panel of automobiles in recent years, the quality of the surface appearance of the galvannealed steel sheet has been particularly required.
In view of reducing the weight of the car body, integral forming of car body parts and reduction in the thickness of the steel sheet have been required, steel sheets having further improved press formability is required more strongly than ever. Therefore, so called IF (Interstitial Free) steel sheets have been used, in which solute carbon and nitrogen are fixed as carbides or nitrides by titanium or the like.
It, however, has been known that use of the IF steel, having titanium added thereto (hereinafter called "Ti-IF steel"), as the substrate of the galvannealed steel sheet involves long streaky patterns (hereinafter called "streaky defect") peculiar to the Ti-added steel being generated, causing surface appearance to deteriorate excessively.
As disclosed in, for example, Japanese Patent Application Laid-Open No. 2-34722, use of the IF steel enables the press formability to be significantly improved as compared with that of conventional low-carbon aluminum-killed steel. Since the prior art is, however, established in order to improve only the press formability, no description has been made about the improvement in the surface appearance. Another prior art paid attention to the surface appearance of the Ti-IF steel is disclosed in Japanese Patent Application Laid-Open No. 2-38550. In view of the streaky defect being caused from an influence of fine crystal grains in the surface layer of the Ti-IF steel, as prevention of the streaky defect, there have been given a method for coarsening the surface crystal grains by pre-annealing or the like and another method for grinding the surface of a hot-rolled steel sheet or a cold-rolled steel sheet. In either case, the additinal step, however, is introduced to the ordinary process for manufacturing the galvannealed steel sheet and, therefore, there arises a problem in that the manufacturing cost becomes too large. Moreover, the method using the pre-annealing process causes the crystal grains to be coarsed deeply in the internal layers, thus resulting in that there will arise problems of deterioration in the workability and rough surface after press stamping. The method using the surface grinding process results in problems of worsening in the manufacturing plant environment due to the ground chips, yield declination and excessive deterioration in the productivity. Therefore, the foregoing method has a problem as a method of manufacturing a galvannealed steel sheet having excellent surface appearance. As a means for improving the strength of the IF steel, addition of solid-solution hardening elements, such as Si, Mn or P into the steel, has been usually employed, as disclosed in Japanese Patent Application Laid-Open No. 2-185949. In the case where the IF steel is strengthened by adding P thereto, long streaky patterns (hereinafter called the "streaky defect") frequently are formed in the rolling direction. The streaky defect sometimes inhibits application of the foregoing steel to the outer body panels of the automobile. Since the foregoing methods have been established to mainly strengthen the steel sheet and to improve the formability, no intention has been made to improve the surface appearance of the steel sheet.
In any event, the foregoing methods are unsatisfactory to improve both the press formability and surface appearance.
As described above, most of the conventional methods have been established to primarily improve the press formability without much attention paid to improvement in the surface appearance. Although there is a method for improving the surface appearance, the cost inevitably rises. Thus, there arises a problem in industrial production.